This invention relates to control of a launched polarization state of a signal beam in coincidence with a principal axis of polarization of an optical fiber. More particularly this invention relates to a method of controlling the launched state, a polarization control system, a polarization control device for controlling the launched state.
As a prior-art polarization control system, a polarization control system for bidirectionally transmitted signal beams is disclosed in U.S. Pat. No. 5,031,998 issued to Takashi Ono, the present inventor, and another hereinafter "th Ono patent". The specification of this United States patent is herein incorporated by reference. In such a polarization control system, polarization controllers are used. It is possible to use as polarization controllers the controller described in an article contributed to the Journal of Lightwave Technology, Volume 9, No. 10 (October 1991), pages 1217 to 1224, entitled "Highly Practical Fiber Squeezer Polarization Controller" by Haruhito Shimizu et. al. The instant inventor is a coauthoer of this work. The, polarization control system of the United States patent is also described in a paper contributed by T. Ono, the present inventor, and three others to the Proceedings of ECOC '90, pages 419 to 422, under the title of "Novel Wideband Common Polarization Control Method for Coherent FDM Transmission System".
Before the prior-art polarization control system was revealed, long-distance transmission of a signal beam through an optical fiber gave rise to accumulation of polarization dispersion in the optical fiber which resulted in an objectionable power penalty. Polarization dispersion results from a difference in a propagation time between two orthogonal principal states, or eigen modes, of an optical fiber. This difference distorts the shape of a received beam and degrades. Reception sensitivity depending upon a launched polarization state with which the signal beam is launched or supplied to the optical fiber. The principal states are described in an article contributed by C. D. Poole et. al. to the Journal of Lightwave Technology, Volume 6, No. 7 (July 1988), pages 1180 to 1190, entitled "Polarization Dispersion and Principal States in a 147 km Undersea Lightwave Cable".
In prior-art polarization control systems, the signal beam and a pilot beam are bidirectionally transmitted through an optical fiber. It both ends of the optical fiber, polarization controllers are connected, and controlled. In this polarization control system, the signal beam is subjected to frequency division multiplexing (FDM) with its launched polarization state kept by the polarization controller to maintain coincidence with a principal axis of polarization defined by the principal states of the fiber. This suppresses adverse effects which would otherwise be imparted to the received beam.
Although subjected to the frequency division multiplexing the signal beam was not frequency modulated in the prior-art polarization control system. Furthermore, the bidirectional transmission is not applicable to an optical fiber which includes an optical amplifier comprising an optical isolator.